Radar devices are equipped on a marine vessel to assist safe and efficient voyages. In order to obtain the optimum radar image, it is necessary to appropriately adjust receiving sensitivity to remove white noise caused inside a radar receiver or clutters including sea-surface reflections, and rain-and-snow reflections at the optimum level. The adjustment of the receiving sensitivity typically is performed by using a gain control device in the radar receiver. In this disclosure, unnecessary signals, such as white noise and clutters are referred to as “noises,” and reflected signals from an object to be detected, such as a ship or a buoy, are referred to as “target object echoes,” to herein distinguish these from the noises.
One of the receiving sensitivity adjustment techniques for the conventional radar devices is disclosed in Japanese Patent No. 3288489, for example. This adjustment technique extracts signals necessary to perform the sensitivity adjustment from the received signals, and counts the number of the extracted signals. Then, the count is compared with a predetermined reference, and a level for gain control signals is adjusted according to the comparison.
The unnecessary signals, such as clutters less likely occur continuously between a present radar image and a past radar image. Thus, the unnecessary signals, such as clutters generated at random are removed based on the correlation of the present radar image and the past radar image to display only echoes from a target object.
However, the levels of white noise may differ depending on radar receivers, and may also vary according to external environmental conditions, such as temperature. The level of clutters due to the sea-surface reflections and the rain-and-snow reflections varies over time, according to oceanic conditions and climates. Therefore, the technique disclosed in Japanese Patent No. 3288489 that compares the count with the predetermined reference value is problematic because it cannot follow the variation in the noise level, as described above.
In addition, because Japanese Patent No. 3288489 counts the signals extracted from the received signals, if necessary signals for performing the sensitivity adjustment are not well extracted, the receiving sensitivity adjustment cannot be appropriately performed due to influence of other signals. For example, when white noise or clutters are intended to be removed, but the extracted signals includes interference waves from other radars or target object echoes, a receiving sensitivity will be lowered more than necessary due to influence of the signal at a strong level. Therefore, there is a problem in which weak target object echoes are not displayed in a display screen.
In order to assist safe cruise in ocean areas, such as areas in proximity to or in harbors, canals or the like where many artificial structures, such as quays, breakwaters, piers, and other vessels exist (hereinafter, these ocean areas are referred to as “port areas”), the radar device is desirable to clearly display artificial structures which exist in proximity to the equipped ship, such as quays, breakwaters, piers, and other ships.
However, the receiving sensitivity that is suitable for removing the noises on the ocean area distant from land and the receiving sensitivity that is suitable for clearly displaying the artificial structures for the port area are not necessarily identical. Therefore, even if the noises on the ocean area are appropriately removable by the conventional automatic gain control disclosed in Japanese Patent No. 3288489, clearly displaying the artificial structures in the port area may be difficult with the same receiving sensitivity.
To remove the unnecessary signals, such as clutters, by performing the correlation process, it is necessary to optimize weights of the present radar image and the past radar image so that only the echoes from target object which is necessary, while removing unnecessary signals, such as clutters are displayed. However, the levels of clutters of the sea-surface reflections and the rain-and-snow reflections vary over time in accordance with oceanic conditions or climates. That is, the levels of clutters may vary even in the same radar-search area. Therefore, the conventional radar device is difficult to perform the correlation process with the optimum weights.